(a) Field of the Invention:
The present invention relates to an apparatus provided with a scintillation crystal and photo-multiplier tubes for detecting the location of a radiation incident to the scintillation crystal.
(b) Description of the prior art:
As the apparatus utilizing detectors for detecting the location of incident radiation which are used in the field of nuclear medicine, there are the well known auto-fluoroscope developed by Bender and Blau, and the scintillation camera developed by Anger. The former radiation-location detector is comprised of a plurality of individual small-size radiation detectors which are closely arranged in a planar or linear fashion, whereas the latter is arranged so that a single large scintillation crystal is embraced by a plurality of photo-multiplier tubes, so that from the magnitude of those outputs of the respective photo-multiplier tubes which are delivered when an incident gamma ray is detected, the location of this incident gamma ray is determined.
Many of those positron type of single photon type emission computed tomography apparatuses which have been put to practice in recent years have the arrangement that a plurality of small-size radiation detectors, especially scintillation counters, are arranged in a ring form to provide a location detector unit. Moreover, there has been proposed an developed concept to arrange such a ring detector in a plurality of layers to perform tomography of a plurality of slices simultaneously. Thus, in order to meet such requirement, it is necessary to arrange a large number of detectors in a narrow space. Also, in order to reduce the number of component parts, and/or to reduce the dimensions of the detectors per se to materialize such congested arrangement of a number of the detectors, there has been proposed to replace individual detectors by a small-size Anger-type location detectors. FIG. 1 is an explanatory perspective view showing the essential portion of such ring detector unit. Reference numeral 1 represents a collimator; 2 an Anger-type location detector; 3, 4 and 5 represent major components contained in a location detector, i.e. a scintillation crystal, photo-multiplier tubes, and a location detecting circuit, respectively. The location detector units 2 are arranged in a ring-like fashion, surrounding an objective for tomography, in the same way as that for the positron-type CT (computed tomography) having an ordinary ring-type arrangement. The collimator 1 is intended for removal of such unnecessary gamma ray that is incident to detector from outside of the intended slices. In FIG. 1, there are shown five doughnut-shape zinc plates. Each space defined by two adjacent zinc plates constitute a slice. There are seen four of such slices. Strictly speaking, arrangement of detectors in a ring form ought to be such that one detector is positioned for each slice, so that there are needed four detectors for four slices. In the conventional arrangement shown in FIG. 1, however, a single scintillation crystal 3 covers the spaces which are for the four scintillation crystals constituting four detectors. However, with a single scintillation crystal as shown, it will be understood that, even when the crystal 3 has detected the presence of incidence of a gamma ray, it is not possible to make cognizance of the location as to in which one of the slices the gamma ray has been detected. Accordingly, in the arrangement illustrated, there are provided two photo-multiplier tubes 4 at the opposite ends of the crystal 3, so that by utilizing the fact that the ratio between the pulse heights of these two photo-multiplier tubes varies depending on the location of scintillation occurring within the crystal where the gamma ray has been detected, a determination is made, by a location detecting circuit 5, of that specific slice at which the detection has been effected. The detection of location of scintillation within the crystal is performed in a manner similar to that by the known Anger-type detector.
While the location detector stated above has an enhanced economical advantage in that, with an increase in the number of slices, the number of the component parts employed can decrease accordingly, it is not possible to avoid the trend of increase in errors in the determination of location of a slice, due to such limitation in the location-resolving ability that is peculiar to the Anger-type detector.